2979 - mu - Soul of a tiny new machine. More thorough tests → More comprehensible and rewrite-friendly software → More resilient society.

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author	Kartik K. Agaram <vc@akkartik.com>	2016-05-19 14:11:10 -0700
committer	Kartik K. Agaram <vc@akkartik.com>	2016-05-19 14:11:10 -0700
commit	dc7947cd7f84711059e00ac593f86f53d14a5a1c (patch)
tree	bc704cb38cbf12dbb3690ffe7aac3bff3e907338 /html/011load.cc.html
parent	02a35f220553b01e09c64383e6c05e86a5c61c15 (diff)
download	mu-dc7947cd7f84711059e00ac593f86f53d14a5a1c.tar.gz

2979

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# Example program showing exceptions built out of delimited continuations. # Since Mu is statically typed, we can't build an all-purpose higher-order # function called 'try'; it wouldn't know how many arguments the function # passed to it needs to take, what their types are, etc. Instead, until Mu # gets macros we'll directly use the continuation primitives. def main [ local-scope foo false/no-exception foo true/raise-exception ] # example showing exception handling def foo raise-exception?:bool [ local-scope load-inputs # To run an instruction of the form: # try f ... # write this: # call-with-continuation-mark 999/exception-tag, f, ... # By convention we reserve tag 999 for exceptions. # # 'f' above may terminate at either a regular 'return' or a 'return-with-continuation-mark'. # We never re-call the continuation returned in the latter case; # its existence merely signals that an exception was raised. # So just treat it as a boolean. # The other inputs and outputs to 'call-with-continuation-mark' depend on # the function it is called with. exception-raised?:bool, err:text, result:num <- call-with-continuation-mark 999/exception-tag, f, raise-exception? { break-if exception-raised? $print [normal exit; result ] result 10/newline } { break-unless exception-raised? $print [error caught: ] err 10/newline } ] # A callee function that can raise an exception has some weird constraints at # the moment. # # The caller's 'call-with-continuation-mark' instruction may return with # either a regular 'return' or a 'return-continuation-until-mark'. # To handle both cases, regular 'return' instructions in the callee must # prepend an extra 0 result, in place of the continuation that may have been # returned. # This change to number of outputs violates our type system, so the call has # to be dynamically typed. The callee can't have a header. def f [ local-scope raise-exception?:bool <- next-input { break-unless raise-exception? # throw/raise: 2 results + implicit continuation (ignoring the continuation tag) return-continuation-until-mark 999/exception-tag, [error will robinson!], 0/unused } # normal return: 3 results including 0 continuation placeholder at start return 0/continuation-placeholder, null/no-error, 34/regular-result ]


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